Vaccinia virus contains a large number of genes that are not essential for replication in tissue culture. The conservation of these genes in vaccinia virus and other numbers of the orthopoxvirus genus, however, signifies that the protein products have an important role in virus-cell or virus-host interactions. The isolation of deletion mutants of vaccinia virus provides a way of identifying the "non-essential" genes. DNA corresponding to a 12,000 base pair deletion was sequenced and 17 open-reading frames were characterized. On of the open reading frames encodes a secretory protein that belongs to the family of complement binding proteins, and has a remarkable similarity to the C4b binding protein. This protein may protect vaccinia virus against host immune responses by regulating the complement cascade. Another open-reading frame, identified because the gene had been duplicated and transposed, encodes a protein that belongs to the family of plasma serum protease inhibitors. A second member of this family was located about 6,000 base pairs away. These proteins may regulate the activity of host serum proteases.